Lipids extracted from bovine amniotic fluid when injected into the superior vena increase the pulmonary arterial pressure of calves. Two separate lipid fractions have been isolated and partially purified, both of which have some pulmonary pressor activities. When the fractions are administered simultaneously, they act synergistically and give a much larger response than the sum of the two individual responses. The two lipid fractions have now been partially purified on columns containing DEAE-cellulose and on plates coated with silicic acid. One fraction behaves chromatographically like cholesterol, but is not cholesterol, because a pure cholesterol is inactive in our in vivo assay system. The other fraction consists of two major lipids, both of which are active and can apparently be used interchangeably. In order to obtain some preliminary information about the structural requirements for the pressor response, we are now proceeding with chemical treatments such as acid and base hydrolyses, oxidations and reductions with various chemical agents and attempts to synthesize derivatives, with such reagents as acetic anhydride, N,O-bis(trimethylsilyl) acetamide and others. Subsequently we will examine the purity of the lipids or their derivatives by gas chromatography and other procedures, prior to obtaining some preliminary evidence about the functional groups and the molecular weight of the lipids by infrared NMR and mass spectrometry. In addition, we will explore the assay of the lipid fraction in an in vivtro system by examining the contraction of isolated smooth muscle such as the aorta in a bath or by superfusion.